Many machine parts have to operate under heavy working load and variable temperature in aggressive environment, which results in their deterioration and calls for protective coating to be applied on such parts. In the field of transport industry and power turbine construction and, particularly, gas turbine manufacturing the surface protection of parts is the most critical. Gas turbine units (GTU) are applied widely in the state-of-the-art technology: aircraft and helicopter's engines, marine gas turbine engines (GTE), power GTU and gas compressor units (GCU). Turbine blades are the major parts that determine reliability, economy in use and the service life of such GTU. Such blades operate under very severe conditions: at elevated temperatures, under considerable fatigue and thermal loads, and in aggressive gas flows containing oxygen, sulfur, vanadium oxides and other chemically-active elements.
Some blades may have internal passages, which are prone to oxidization, especially if made of currently employed high temperature superalloys with low content of chromium. During use, the coating undergoes cracking, flaking, diffusion dispersal, corrosion and erosion attack, and the chemical and phase compositions change in the surface layers. As a result, durability of blade decreases and such blades have to be laid off.
Turbine blades are manufactured of expensive superalloys by employing a complicated technology, for instance, oriented crystallization or monocrystal casting, so their price is extremely high. Therefore, protective coating technologies, which improve durability and service life of such blades, provide great economical benefits and significant technical advantages.
Well-known are the protective coating methods, when aluminide coatings or Me—Cr—Al—Y coatings are applied on superalloys (U.S. Pat. Nos. 3,542,530; 3,544,348; 3,918,139; 3,961,098; 3,928,026; 3,993,454; 4,000,507; 4,132,816; 4,034,142); aluminide coatings alloyed with noble metals Pt, Ro, Pd (U.S. Pat. No. 3,819,338); the method for protection of gas turbine blades from high temperature corrosion (Russian Federation Patent No. 2033474), which includes vacuum deposition of two layers: a Me—Cr—Al—Y layer and a layer of aluminum-based alloy, with subsequent vacuum annealing.
Diffusion methods for powder and gas vapor deposition are known to create aluminum intermetallic coatings, which, while having quite high heat-resistance, at the same time possess low resistance to thermal stresses and to corrosion in the chemically aggressive environment of combustion materials. Coatings alloyed with noble metals are expensive and their use is not always economically sound. Slip powder technologies can not provide coatings, which would be uniform in thickness, and the density of such coatings is not sufficient. Aluminide coatings, too, have high thermal conductivity and insufficient correlation between linear expansion coefficients in oxide ceramics layers.
Frequently occurring fault with multi-layer coatings of Me—Cr—Al—Y and their modifications is that they do not provide long enough service life of a machine component either due to insufficient heat-resistance or as a result of flaking and corrosion of the coating during use.
One has to face really serious problems at the stage of preparing the surface of superalloy-made components for coating by vacuum plasma deposition in order to obtain high adhesion of the coating with the substrate metal.
U.S. Pat. No. 4,080,486 describes a coating method by diffusion powder deposition of aluminide onto the surfaces of components, following the deposition of vacuum plasma Me—Cr—Al—Y coating. This patent neither fully uses all means to offer maximum resistance to gas corrosion, nor provides protection for internal passages of cooled cavities.
European patent EP 0-897-996A describes a complex coating for heat-resistant nickel- or cobalt-based alloy matrixes, which, for instance, would be used for gas turbine engine blades. The said patented coating comprises a MeCrAlY type of compound, wherein Me is the element selected from the group consisting of iron, nickel and cobalt. The said coating is subjected to aluminizing by means of gaseous phase diffusion saturation, and also includes the formation of a diffusion aluminide coating over the MeCrAlY system on the outer surfaces and deposition of an aluminide coating on the internal surfaces of a coated component, both with and without platinum bondcoat applied.
The said method is the closest to the proposed invention, however, it leaves room for further improvement of resistance to flaking and erosion, for enhancing thermal fatigue characteristics of coated alloys and for improvement of sulfur corrosion resistance.
Also known is Russian Federation Patent No. 2073742 that describes a method for protecting coating composed of multi-component Ni—Cr—Al—Ta—Y alloy with subsequent chrome aluminizing by powder technique and quenching after retention in vacuum furnace at 1200° C.
Even with this method, there is still an opportunity to further improve performance characteristics of coatings and durability of coated parts.
Russian Federation Patent No. 2113538 describes repetitive pulse ion plasma treatment of parts and a device for such treatment, which incorporates an arrangement for continuous plasma generation, and the radiation doze is controlled by altering the repetition rate and pulse duration and by varying the source/part gap. This patent does not cover the issues of coating formation nor does it address the capabilities of patented device to improve the coating technology.
Russian Federation Patent No. 2029796 describes a method for a combined ion-plasma treatment, which implies surface treatment of parts, in particular, high-speed steel plates, by directed flow of particles. This directed flow is meant to provide partial destruction of brittle passivating phases in the surface, which affords better adhesion between the coating and substrate material.
The present invention is to address the problem of durability and reliability of machine components, particularly, parts made of iron-, nickel and cobalt-based superalloys with complex high-temperature protective coating, and, more specifically, to cover modifications of complex protective coating methods developed for such parts, especially for gas turbine blades and vanes.